1. Technical Field
The present invention relates to a semiconductor device including a metal interconnection and a method for forming the metal interconnection.
2. Description of the Related Art
There are great demands for device application technologies using various interconnection materials to produce high-speed and high-integration semiconductor devices. A semiconductor manufacturing process is classified into either an FEOL (front end of the line) process for forming a transistor on a silicon substrate or a BEOL (back end of the line) process for forming metal interconnections. The process for forming metal interconnections refers to a process for electrically connecting devices fabricated through a planar process to each other to obtain an integrated circuit. That is, contact holes and via holes are formed through the interconnection forming process to connect semiconductor devices with metal interconnection layers and to electrically connect interconnection layers with each other.
Current interconnection technology for forming metal interconnections mainly includes an interlayer dielectric layer forming process, a silicide forming process, a barrier metal forming process, and a metal interconnection forming process. The interconnection technology is used to minimize electric delay and loss of signals when the signals are transmitted between devices while allowing the devices to have superior response speed and reliability.
The interlayer dielectric layer mainly includes a silicon oxide (SiO2) layer, which may be deposited through typical deposition processes, and the metal interconnection material includes tungsten (W) or aluminum (Al). If tungsten is used for the metal interconnection, a barrier metal layer having a dual-layer structure comprising titanium/titanium nitride layers (Ti/TiN layers) must be formed between the tungsten metal interconnection and the interlayer dielectric layer to improve the adhesion property between the tungsten metal interconnection and the interlayer dielectric layer and to prevent metal diffusion. In the interconnection process, Ti/TiN/W/Ti/TiN layers are stacked on the oxide layer, which serves as an interlayer dielectric layer, and then a metal interconnection is formed through a photolithography process. The above process may repeat several times to form a dual-layer metal interconnection or a multi-layer metal interconnection.
During the interconnection forming process, a barrier metal layer is prepared in the form of a dual-layer structure consisting of a titanium (Ti) layer and a titanium nitride (TiN) layer producing superior step coverage and anti-diffusion characteristics. The Ti layer improves the step coverage at the bottom of a hole, thereby reducing the contact resistance. The Ti layer is formed through an ionized metal plasma (IMP) process. In addition, the TiN layer is formed through a chemical vapor deposition (CVD) process. That is, the TiN layer is formed using a CVD process performed at a temperature of about 400° C. such that tetrakis-dimethyl-amido-titan (TDMAT) can be thermally decomposed, thereby forming a porous, amorphous TiN layer. Then, a hydrogen/nitrogen (H2/N2) plasma treatment is performed, thereby forming a crystalline TiN layer. The TiN layer formed through the CVD process produces superior step coverage, so that the TiN layer is used as an adhesion layer for tungsten (W) while serving as a diffusion barrier to prevent fluorine penetration of tungsten hexafluoride (WF6).
Tungsten (W), which is typically a material for metal interconnections, is deposited by a CVD process in a nucleation step and then a bulk deposition step. In the nucleation step, WF6 reacts with silicon hydride (silane (SiH4)), thereby forming a tungsten crystal nucleus as expressed by the formula2WF6+3SiH4=W+3SiF4+6H2.In the bulk deposition step, WF6 reacts with hydrogen (H2), thereby forming tungsten as expressed by the formulaWF6+3H2=W+6HF.
However, the above metal interconnection process has the following problems. Since the Ti layer used for the barrier metal layer is deposited through a sputtering process, an overhang 16b is formed in the vicinity of an inlet of a contact hole 15 as shown in FIG. 1. Overhang 16b becomes enlarged as the thickness of a Ti layer 16 increases. Then, a TiN layer 17, having an amorphous phase, is formed through the CVD process. The amorphous phase of TiN layer 17 can be converted into the crystalline phase through H2/N2 plasma treatment. Since TiN layer 17 represents the superior step coverage characteristics, TiN layer 17 is deposited along a profile of Ti layer 16, so overhang 16b still remains in the vicinity of the inlet of the contact hole. In addition, when performing the plasma treatment process, plasma is concentrated onto overhang 16b, so that the structure of TiN layer 17 is changed from the porous amorphous structure to the columnar structure. In this case, micro cracks may occur in TiN layer 17, causing deformation of TiN layer 17. Furthermore, since the plasma rarely reaches a shadow area 17d formed at the bottom of overhang 16b, the porous amorphous TiN layer may not be changed into the crystalline TiN layer. In this state, if the tungsten deposition process is performed, fluorine diffusion may occur at shadow area 17d formed at the bottom of overhang 16b. That is, fluorine penetrates through the porous amorphous TiN layer 17, so that fluorine makes contact with Ti layer 16 formed below TiN layer 17, thereby causing a volcano defect 17c as shown in FIG. 2.
In addition, hydrogen generated during the nucleation step reacts with WF6, thereby forming tungsten (W) and hydrogen fluoride (HF). Thus, “HF+F” may diffuse into the barrier metal layer and makes contact with Ti layer 16, thereby forming TiFx as expressed by the formulaTi+(HF or F)=TiFx[TiF3(solid) or TiF4(gas)]+H2.Since TiF4 is a volatile gas, TiF4 may lift TiN layer 17 when it is evaporated, so that a TiN lifting 17b may occur as shown in FIG. 3B.
In the case of a serious TiN lifting 17d (see FIG. 3A), a seam formed in contact hole 15 has a small size as shown in FIG. 3C. However, in the case of a weak TiN lifting 17f (see FIG. 3A), TiN lifting 17b may block the inlet of contact hole 15, so that a seam 18b of a large size is formed in contact hole 15 when tungsten 18 has been deposited, thereby forming the volcano defect 17c as shown in FIG. 3D.
As shown in FIG. 4A, such a volcano defect causes a “contact not fill” phenomenon 18c. In addition, when a tungsten polishing process is performed, a portion with the volcano defect has a lower removal rate than that of a portion without the volcano defect, so that tungsten residues 18d are formed as shown in FIG. 4B. For this reason, over polishing is carried out in order to remove the tungsten residues 18d. However, over polishing may cause damage to an oxide layer 14 as shown in FIG. 4C.